Improve the test for dirfd(), some systems #define it rather than have
an actual function.
diff --git a/pyconfig.h.in b/pyconfig.h.in
index 9360981..8e8f319 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -178,7 +178,7 @@
    */
 #undef HAVE_DIRENT_H
 
-/* Define to 1 if you have the `dirfd' function. */
+/* Define if you have the 'dirfd' function or macro. */
 #undef HAVE_DIRFD
 
 /* Define to 1 if you have the <dlfcn.h> header file. */